Conventional power transmission devices for transmitting power to vehicle wheels are disclosed in JP Patent Publication 2005-7914A and JP Patent Publication 5-332401A. The one disclosed in JP Patent Publication 2005-7914A, which is an in-wheel motor driving unit, comprises an electric motor for generating a driving force, a wheel shaft to which the body of a wheel is connected, and a speed reducer for reducing and transmitting the rotation of the electric motor to the wheel shaft. The speed reducer is a gear speed reducer with parallel shafts in which a plurality of gears having different numbers of teeth are combined.
JP Patent Publication 5-332401A discloses a speed reducer for use in an electric vehicle, which includes planetary gear speed reducer units provided in two stages between the rotor of an electric motor and wheel hubs. The output of the second-stage planetary gear speed reducer unit is distributed to right and left wheels under springs.
Because the in-wheel motor driving unit disclosed in JP Patent Publication 2005-7914A eliminates the necessity for large-sized power transmission mechanisms such as propeller shafts and differentials are not necessary, use of such driving units can reduce the weight and size of the vehicle. But the reduction ratio provided by the gear speed reducer with parallel shafts is only about 1/2 to 1/3, which is insufficient as a speed reducer mounted in an in-wheel motor driving unit. Also, because such a speed reducer is heavyweight, the use of such speed reducers adds to the weight of the vehicle under the springs, thereby deteriorating the drive feel. Thus, this in-wheel motor driving unit is not yet put to practical use.
Although the planetary gear speed reducer units of the speed reducer for an electric vehicle disclosed in JP Patent Publication 5-332401A provide a higher reduction ratio than gear speed reducers with parallel shafts, their reduction ratio is still insufficient for use in an in-wheel motor driving unit. In order to obtain a sufficiently high reduction ratio, planetary gear speed reducing units, which each comprise a sun gear, a ring gear, pinion gears and a carrier for the pinion gears, have to be provided in many stages. This increases the number of parts of the speed reducer and makes it difficult to provide a compact speed reducer. That is, such a speed reducer tends to be heavyweight and large in size.